Organic polyisocyanates have long been used as binders for lignocellulosic materials in the manufacture of sheets or molded bodies, such as chipboard, fiberboard and plywood. Although such organic polyisocyanates have excellent adhesive properties, they have the disadvantage of causing severe sticking of the lignocellulosic materials to metal surfaces contacted during pressing, such as the platens of the press. Such sticking may harm the end product and the necessary cleaning of the metal surfaces is both time-consuming and costly.
Attempts have been made to overcome the adhesion problem caused by polyisocyanate binders. For example, efforts have been made to use the polyisocyanates only in the core of the molded products and to replace the polyisocyanates with phenolic resin binders at the surface. Phenolic resin binders cause less adhesion of the lignocellulosic material to the metal surfaces of the press, however, they are inferior to isocyanates in their adhesive properties and have a relatively slow rate of reaction.
Also, attempts have been made to improve the release characteristics of organic polyisocyanate binders in order to decrease or eliminate the adhesion problem through the use of internal and external release agents. Internal release agents used within the polyisocyanate binder to decrease the adhesion problem include organic phosphates, as disclosed in U.S. Pat. No. 4,257,996. External release agents include metallic soaps, such as those disclosed in U.S. Pat. No. 4,110,397.
Various acids or salts of acids have also been incorporated into polyisocyanate binders in an attempt to increase the release capabilities thereof. For example, U.S. Pat. No. 3,870,665 discloses a process for molding lignocellulosic materials with a polyisocyanate. A catalyst for promoting isocyanurate formation is included within the isocyanate to effect release. The catalyst may be a fatty acid, such as a coconut fatty acid. U.S. Pat. Nos. 4,905,518 and 4,609,513 disclose liquid binders comprising polyisocyanates and acids or acid esters. U.S. Pat. Nos. 4,933,232 and 4,772,442 disclose a process for producing shaped articles utilizing a binder/release system comprising an isocyanate and a polymeric fatty acid. Although effective in improving the release characteristics of the organic polyisocyanate, the inclusion of acids such as fatty acids directly into the isocyanate binder has been unsatisfactory due to stability problems. Namely, when used in the amounts necessary to provide adequate release activity, the acids have been found to react directly and immediately with the polyisocyanate resulting in an unstable and unusable system.
Various waxes have also been combined with polyisocyanate binders to provide improved release. For example, U.S. Pat. No. 4,396,673 discloses the use of a mixture of an organic polyisocyanate and a wax release agent to prepare sheets or molded bodies from lignocellulosic materials. U.S. Pat. No. 4,431,455 discloses the use of an organic polyisocyanate and a mixture of a wax and liquid ester to promote release of lignocellulosic material from press plates. However, such approaches have proven to be unsatisfactory as they require the use of special solvents and/or emulsifiable isocyanates. In general, wax release agents are disadvantageous as a layer of the wax is always left on the surface of the composite which can adversely affect the physical characteristics of the composite.